Illinois legalized recreational cannabis in 2020. The per-se DUI threshold of 5 ng/mL THC is doing more legal work than the science supports.
Illinois made recreational cannabis legal on January 1, 2020. At the same time, the legislature wrote a per-se DUI standard into 625 ILCS 5/11-501: 5 nanograms of delta-9 THC per milliliter of whole blood, or 10 ng/mL in another bodily substance. Hit that number and you're guilty of DUI, even if you weren't actually impaired at the moment of driving.
That standard is doing a lot of legal work. It's also doing a lot of scientific work that the underlying research doesn't support.
The 5 ng/mL line
Unlike Missouri (which has no per-se cannabis threshold and uses totality of the circumstances), Illinois lets the prosecution prove DUI two ways:
- Actual impairment. Observation, FSTs, DRE evaluation, driving behavior.
- Per-se violation. Chemical test result at or above 5 ng/mL THC in blood (or 10 ng/mL in urine/oral fluid).
Either route is enough for a conviction. The per-se path is what makes Illinois cannabis DUI prosecutions easier than Missouri's.
Why 5 ng/mL is scientifically shaky
Frequent cannabis users. Including medical patients. Can show baseline THC levels above 5 ng/mL for hours or even days after their last use. Studies that try to correlate blood THC level with driving impairment generally find a poor relationship. NIDA, AAA, and the National Safety Council have all published research questioning whether any single THC threshold is a valid impairment marker.
That's not a defense by itself. Illinois law is the law. But it's the foundation of expert challenges in serious cannabis DUI cases.
[Refusing the chemical test](/glossary/implied-consent)
Illinois treats refusal of a chemical test as a Statutory Summary Suspension. Automatic, separate from the criminal case. First refusal: 12 months. Subsequent refusal: 3 years. You have 46 days from the notice to file a Petition to Rescind.
Refusal eliminates the per-se evidence but doesn't end the case. The state can still prosecute on actual-impairment evidence (officer observations, FSTs, DRE protocol).
What gets challenged on a cannabis DUI
- The stop and detention. Basis for the initial stop, length of detention while waiting for a DRE.
- Field sobriety tests. Validated for alcohol, not THC. Even NHTSA acknowledges limitations.
- DRE evaluation. Admissibility under Frye, training records, validation rates.
- Blood draw. Collection procedures, warrant compliance, lab handling.
- The 5 ng/mL standard itself. Expert challenges to the scientific foundation.
The Petition to Rescind has a 46-day clock from the date of the notice. Not from the date of arrest. Calendar it the day you get the paperwork. Missing this deadline waives meaningful rights the criminal case alone won't recover.
Each cannabis DUI case turns on its specific facts: what testing was done, when, how, and what the officer documented. This is general background, not legal advice.